Why so many mammals see fewer colours than birds do
Early mammals escaped predators and competition by switching to night-time activity, and paid for it with their eyes: they lost two of four colour-sensing cone pigments. Most mammals are still dichromats today. When our primate ancestors returned to the daylight, they evolved three-colour vision again, a lasting trace of that long nocturnal detour.
Diurnal animals are active by day and rest at night, as opposed to nocturnal creatures, twilight-loving crepuscular ones and cathemeral animals that come and go at any hour. Most primates, humans included, are diurnal, as are many birds and reptiles. Plants follow the pattern too, timing their flowers for their pollinators: sunflowers open by day for bees, while the night-blooming cereus waits for large sphinx moths. Baobab flowers, pollinated by fruit bats, open in late afternoon and are dead within a day.
Daytime living keeps evolving afresh. About 430 gecko species, members of a group long assumed to be nocturnal, are now active by day, the result of some 20 separate transitions. Small rodents such as the Nile grass rat have made the same shift.
Light is the master signal. It acts on the suprachiasmatic nucleus, a small region of the hypothalamus that runs the body clock and triggers hormone cascades. Light can also override that clock, an effect called masking. Put a diurnal Nile grass rat and a nocturnal mouse under identical lighting, and the rat grows livelier while the mouse slows down. Owl monkeys in South America's Gran Chaco are busier on moonlit nights and less active the following day; on dark nights they struggle to forage and must feed in daylight instead.
Temperature and hunger matter too. Nocturnal animals lose heat during cool nights, and under the circadian thermo-energetics hypothesis, an animal burning more energy than it takes in should shift its activity into the day. Laboratory mice given enough cold and hunger stress did exactly that. Human life has its own daily rhythm: commuting peaks, red-eye flights, and websites that schedule maintenance for their quietest hours.
Source: Diurnality
